1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic printer, a copying machine, and the like.
2. Description of the Related Art
A corona charging method for charging a surface of a unidirectionally rotating photosensitive member with a predetermined potential V0 by corona discharge is used widely in an image forming apparatus such as an electrophotographic printer, a copying machine, and the like. Although the corona charging method has excellent characteristic of electrostatically charging the photosensitive member uniformly, the corona charging method has a problem that the method requires a countermeasure for a great deal of ozone generated at the time of generation of corona because a high DC voltage of about 4to 6kV is used.
For this reason, there has been proposed a contact charging method in which a desired charged potential can be obtained by use of a relatively low voltage while the amount of ozone generated is extremely small.
The contact charging method is a method for electrostatically charging the surface of the photosensitive member by applying a relatively low voltage to a charging means which is disposed so as to come into direct contact with the photosensitive member.
The contact charging method is classified into an AC type and a DC type. In the AC type, an AC voltage or an AC/DC superposed voltage is applied to the charging means as disclosed in JP-B-3-52058. In the DC type, a DC voltage is applied to the charging means as disclosed. in JP-A-6-348112 and JP-A-10-198132.
The AC type achieves uniform electrostatic charging in spite of generation of a very small amount of ozone (approximately 1/10 to 1/100 compared with the corona charging method) and has a high capacity for removing the potential hysteresis of the photosensitive member. For this reason, the AC type has an advantage in that the scale of configuration of the electrophotographic apparatus as a whole can be reduced without necessity of any static eliminating process before the electrostatic charging process but has a disadvantage in that vibratory noise is generated in a nip portion by an AC electric field because an AC voltage is used.
On the other hand, the DC type is apt to make electrostatic charging so uneven that stripe-like electrostatic charging irregularity reaching 200 mm at maximum may be generated in a direction perpendicular to a direction of movement of a charged surface. For this reason, there is a problem that a white-striped image defect (the phenomenon that white stripes appear in solid black or half tone) occurs in the case of a charged area development, and that a black-striped image defect occurs in the case of a discharged area development.
In the contact charging method, the photosensitive member is electrostatically charged on the basis of gap discharge at gaps on opposite sides of the position of contact between the photosensitive member and a charging roller. It is difficult to charge the photosensitive member evenly because a large number of factors such as the relative dielectric constant of the photosensitive member, the applied voltage, the film thickness,and the like have relation to the gap discharge phenomenon in terms of characteristic of the charging mechanism.
To solve the problem, several proposals have been made.
For example, JP-A-6-348112 has reported the knowledge that selection of dark potential of the photosensitive member in a range from 300 to 650 V permits uniform charging. According to JP-A-6-348112, there is a report that when the dark potential of the photosensitive member is selected to be not higher than 650 V, an air gap can be controlled within a certain range to stabilize charging to thereby achieve a success in uniform charging free from any striped image in terms of total charging characteristic in the same manner as in the case where a pulsating voltage is applied. It is however indicated that contrast to bright potential cannot be taken so that a problem of reduction in density and a problem of fogging or the like occur in charged area development and discharged area development respectively when the dark potential of the photosensitive member is not higher than 300 V.
To pay attention to the fact that a discharge is not stabilized when discharge is performed at gaps on both upstream and downstream sides of the position of contact between the photosensitive member and the charging roller, there has been also proposed a method in which charging is performed only at the downstream gap.
In this method, it is however necessary to accurately irradiate the upstream gap portion with a large quantity of light in order to perfectly eliminate static electricity at the gap located in the upstream side in the direction of movement of the photosensitive member. A photo carrier generated in the photosensitive member by light irradiation may remain after passage through the nip portion to thereby eliminate static electricity at the upstream gap. There is a problem that charging efficiency becomes down.
On the other hand, JP-A-10-198132 has disclosed a method in which the surface of the photosensitive member is charged with a predetermined potential by discharge at the upstream gap portion so that necessary charging is performed at only the upstream gap. According to JP-A-10-198132, there is a disclosure that uniform charging can be performed when a pre-exposure means for clearing the surface potential of the photosensitive member is provided on the upstream side of the position of contact between the charging roller and the photosensitive member while satisfying L/v≧=τ in which L (mm) is the distance between the most downstream point of an irradiation region due to the pre-exposure means and the charging start point of the charging roller, v (mm/sec) is the moving velocity of the photosensitive member, and τ is the lifetime of the photo carrier generated in the photosensitive member by the pre-exposure means.